The present invention is directed to the electroplating of nickel-iron alloys, particularly the nickel-rich alloys employed as coatings for corrosion protection and as magnetic thin films in the head of a computer hard disk drive. The invention encompasses electroplating baths or solutions, the use of complexing agents in these baths, and the methods which employ these baths.
Nickel-iron (Ni-Fe) alloys exhibit a variety of useful properties. One particular alloy, known as Permalloy (78% Ni, 22% Fe), finds extensive use as a soft magnetic material in the head of a computer hard disk drive. The "thin film head" now dominates all other head designs. The Permalloy components of a thin film head are produced by an electrodeposition process reported in the open literature in 1962 by Honeywell scientists.
The addition of increasing amounts of nickel to a nickel-iron alloy produces a dramatic improvement in its corrosion resistance. Thus, electroplated nickel-iron alloys that are rich in nickel have been considered for use as decorative coatings on steel implements. The ability to produce such coatings in the "bright" condition, in which the deposit is smooth on a microscopic scale and thus does not require polishing after the deposition, makes this type of deposit desirable from an aesthetic as well as economic perspective.
Since the standard reduction potentials of nickel and iron are similar (-0.25 and -044, respectively), it might be imagined that the electrodeposition of this composition would be straightforward. However, compositional control is complicated by the phenomenon of anomalous codeposition (AC), in which the more active metal (iron) deposits preferentially to the more noble metal (nickel). In fact, in order to achieve a Ni/Fe mass ratio of 3.5 in the deposit (for Permalloy), a Ni/Fe ratio in excess of 40 is typically maintained in the bath.
The first explanation for the anomalous codeposition observed in the nickel-iron system was provided by Dahms and Croll in 1965 (1). Their data indicate that ferrous ion in the bath produces an increase in the deposition overpotential for nickel. It was recognized at the time that hydrogen ion is reduced at the cathode, and that this half-reaction produces an increase in the pH at the surface of the deposit. Thus, Dahms and Croll attributed the AC to the formation of an Fe(OH).sub.2 precipitate film on the deposit. Some years later Hessami and Tobias (2) suggested that the increase in cathode pH was only sufficient to produce the soluble iron hydrolysis product FeOH.sup.+. A mathematical model of anomalous codeposition based on the adsorption of this species on the cathode successfully predicted Ni-Fe deposit compositions under certain sets of conditions. Shortly after the Hessami-Tobias model was published, Deligianni and Romankiw (3) experimentally verified that the increase in cathode pH was modest. Furthermore, it was noted that the pH increase was similar to that observed with ferric ion solutions, suggesting that ferric ion (present in the bath as the result of the air oxidation of ferrous ion) was controlling the cathode pH. Thus, they attributed anomalous codeposition in the nickel-iron system to the adsorption of the hydrolysis products of ferric ion.
In the current electrodeposition bath utilized in the commercial production of nickel-iron (Ni-Fe) thin films, the nickel and ferrous ions are weakly complexed by chloride and/or sulfate ions. Under these conditions, the iron deposits preferentially to the nickel, so much so that, in order to produce a deposit composition of 22% iron and 78% nickel (the Permalloy composition), the concentration of ferrous ion in the solution must be precisely maintained at a low concentration (e.g. 0.0050M). Precise maintenance of this ferrous ion concentration is made difficult by the consumption of iron in the deposition, and by its oxidation (to ferric ion), precipitation (as ferric hydroxide) and subsequent removal by filtration. The actual iron content achieved with such a bath is quite sensitive to the iron concentration in the bath. The deposit composition is also sensitive to the applied current density under these conditions (FIG. 3). With careful control of the bath chemistry and the current applied to the electroplating cell, the composition provided by this system can still vary by up to 10%.
From the discussion above it is clear that it would be desirable to possess an electroplating system capable of producing nickel-rich nickel-iron alloys that 1) allowed for higher concentrations of iron to be employed in the bath, 2) limited the sensitivity of the deposit composition to the concentration of iron in the bath, and 3) limited the sensitivity of the deposit composition to the applied current density. Thus, the patent record was searched for references to nickel-iron electroplating, and the use of complexing agents and/or compounds, in nickel-iron electroplating.
The patents discussed below point out that two very different attitudes exist with respect to the electroplating of nickel-iron alloys. For the production of thin films for magnetic applications, the electroplating bath tends to be as simple as possible, and the morphology (roughness) of the deposit, while important, is secondary to the composition. For the thicker deposits used to protect a steel substrate from corrosion, freedom from pinholes and the surface morphology (roughness) of the deposit are most important.
In the electrodeposition of magnetic thin films, control of the bath chemistry is of paramount importance. Additives are viewed with trepidation, unless their action is absolutely essential and their electrochemical behavior is reasonably well understood. Boric acid (which limits the inclusion of hydroxides in the deposit), saccharin (which reduces tensile stresses in the deposit) and a wetting agent such as sodium lauryl sulfate (which assists in the detachment of hydrogen gas bubbles from the deposit) are considered essential. Castellani, Powers and Romankiw (U.S. Pat. No. 4,102,756) state that "complexing agents are to be avoided," but nonetheless note that citrate, tartrate, oxalate and phosphate may be used. Since all of these complexing agents are known to form stable complexes with ferric ion, it is clear that the primary effect of their addition would be to limit the precipitation of ferric hydroxide.
Two other patents, U.S. Pat. Nos. 4,239,587 and 4,780,781, relate to the electrolytic production of magnetic thin films. In U.S. Pat. No. 4,239,587 the use of a bath to produce a relatively high iron content in the deposit is described. The purpose of the higher iron content is to enhance the ability to etch the material during further processing of the head. The bath used in producing these deposits is prepared with iron (II) ammonium sulfate. As seen below, the ammonium ion present in this bath has little or no effect on the deposit composition.
In U.S. Pat. No. 4,780,781 the deposition of Co/Ni/Fe thin films for micro-magnetic applications is described. One of the baths in this invention contains a very small amount (0.1 g/L) of dimethylamine-borane. While not stated explicitly, it would appear that the purpose of this additive is the introduction of a small amount of boron into the deposit. In U.S. Pat. Nos. 4,228,201 and 5,403,650, dimethylamine-borane is listed as a reducing agent in the "electroless" deposition of nickel (this process is explained in detail below). In these two cases it is clear that dimethylamine-borane is not behaving as a complexing agent.
For the electroplating of nickel-iron coatings for decoration and/or corrosion protection, the shininess (absence of roughness) of the deposit is of primary importance. When produced from "simple" baths, nickel-iron alloy deposits are gray colored as the result of a surface that is rough on a microscopic scale. To overcome this undesirable appearance, organic additives known as "brighteners" are added to the bath. These compounds act by adsorbing onto the surface, inhibiting the growth of existing crystallites, and therefore enhancing the rate of nucleation of new crystallites. The resultant deposit is fine grained and microscopically smooth, or bright.
U.S. Pat. Nos. 5,194,140, 4,434,030, 4,450,051, 4,179,343, 3,974,044, 4,134,802, 4,002,543, 3,878,067, and 4,421,611 describe a compound or mixture of compounds that promote the brightness (and/or limit the porosity) of a nickel-iron coating. The chemistry of these baths is quite complex. Ethylenediamine (EDA) is mentioned in some of these patents, but it is typically reacted with other compounds (to yield higher molecular weight species that serve as a "brightener") prior to its addition to the bath. It would appear that such additives do not affect the Ni/Fe mass ratio of the deposit by enhancing the rate of nickel deposition, because it is recommended in the patents that the baths contain 10-40 times more nickel than iron (II) ions.
U.S. Pat. Nos. 5,194,140 and 5,417,840 indicate that EDA, as well as higher molecular weight polyamines and hydroxylated polyamines, may be used as complexing agents in the electroplating of zinc-cobalt and zinc-nickel alloys (both of which also serve as coatings for corrosion protection). The baths from which these alloys are produced are alkaline; if the metal ions are not complexed, they will precipitate as hydroxides. Thus, the presence of these complexing agents is necessary to maintain the metals in solution.
U.S. Pat. Nos. 3,045,334 and 5,403,650 relate to the "electroless" or chemical deposition of nickel. In these systems, reduction of nickel ion to nickel metal is caused not by the application of a current, but by reaction with a reducing agent present in the bath. To control the rate of this redox reaction (and to have it occur only on surfaces that are catalytic for it), the nickel in the bath must be in the complexed form. Thus, complexing agents are an important component of any electroless plating bath (see also U.S. Pat. No. 5,258,200). EDA and diethylenetriamine (DETA) are both listed as complexing agents for use in electroless nickel baths. The action of EDA in this capacity was examined by A. Vaskelis et al. in 1986, as reported in Surface and Coatings Technology (4). They note that EDA reduces the decrease in pH that occurs at the deposition surface during the course of the deposition, and as a result increases the rate of metal build-up on the surface. However, in this process only nickel is being deposited.
The use of the organic amine 1,2-diaminopropane in a bath for the electroplating of palladium to provide a corrosion-resistant coating for interconnects is described in U.S. Pat. No. 5,178,745. The bath is referred to as a strike bath, meaning that it is suitable for electroplating on metals more "active" than palladium. Immersion of such a metal in a solution containing palladium will result in a deposit even without the application of a current; the palladium and the substrate metal undergo what is known as a "displacement" reaction. Such displacement deposits do not have the physical integrity required for deposits used in magnetic thin film or corrosion protection applications. The organic amine in this bath serves to shift the deposition potential to more negative values, so that displacement becomes impossible.
Hence, there exists a need for an improved electroplating bath for producing nickel-iron (Ni-Fe) thin films, alloys, products, and methods.